In magnetic resonance imaging (MRI), some received signals may be useful for imaging a structure (e.g., blood vessel wall, heart valve). Other received signals may not be useful for imaging that structure. For example, signal from a blood vessel wall may be useful for imaging the blood vessel wall while signal from blood flowing through that blood vessel may not be useful for imaging the blood vessel wall. Indeed, it may be desirable to suppress signal from blood flowing through a blood vessel to facilitate acquiring a clear image of the blood vessel wall. Similarly, it may be desirable to suppress signal from blood flowing through a heart valve to facilitate acquiring a clear magnetic resonance (MR) image of the heart valve.
Several conventional approaches have attempted to image tissue and/or structures in the presence of flowing blood. These may be referred to as “dark blood” imaging techniques. In dark blood imaging, signal is received from the structure of interest. However, signal is not received from blood that is near the structure of interest or signal that is received from the blood has properties that make those signals distinguishable from signals from the structure of interest.
One previous dark blood imaging technique is described in “Dark-Blood True-FISP Imaging Using Dual Steady States”, Duerk, et al., Proc. Intl. Soc. Mag. Reson. Med 11 (2004). This technique involved establishing separate steady states in and out of a plane of interest. This technique produced excellent suppression of inflowing spins and consistent TrueFISP contrast within a slice. However, the minimum repetition time (TR) of the sequence was extended by at least 1.5 ms over a standard TrueFISP sequence to permit inclusion of a slab-selection radio frequency (RF) pulse and an extra gradient area. Another previous technique is described in “A Radial Steady State Free Precession Approach”, Duerk, et al., Proc. Intl. Soc. Mag. Reson. Med. 11 (2004). This technique involved a radial steady state free precession (SSFP) sequence with random amplitude velocity encoding gradients applied prior to data acquisition. Thus, a standard TrueFISP sequence was modified by adding a random bipolar gradient prior to radial SSFP data acquisition in each TR. This increased the TR. Thus, previous attempts to produce dark blood (DB) contrast in steady state sequences have typically lengthened the repetition time (TR) and/or disturbed the steady state of a slice to be acquired. The longer TR may introduce and/or exacerbate issues associated with eddy current artifacts, increased off-resonance banding artifacts, and so on.
In MRI, different types of contrast can be generated by applying different radio frequency (RF) and magnetic field gradient pulses to a subject area. These different types of contrast can be used to distinguish between items. Additionally, different types of contrast may be available because a subject may be comprised of different materials. For example, a subject area may include tissue and blood. The tissue may include fat and water. Different materials to which different RF and magnetic field gradient pulses are applied may produce different magnetic resonance (MR) signals. Once again, the different MR signals may be used to distinguish between items. Dark blood imaging involves either acquiring a first type of signal from an item (e.g., blood vessel wall) near blood and a second type of signal from the blood near the item or acquiring signal from an item (e.g., heart valve) near blood while preventing the blood near the item from generating a signal. The blood near an item may not be static. For example, blood may be flowing in a blood vessel to be imaged. Similarly, blood may be moving through a valve to be imaged. Either suppressing the signal from the blood or making the item to be imaged and the blood near the item to be imaged have different MR signals facilitate acquiring better MR images of the item to be imaged.